- A circuit board that has sockets which allow other circuit boards to be plugged into it. It may or may not be processor-controlled, which would make it active or passive, respectively.
- The main circuit board of a computer, containing the connectors necessary to attach additional boards. It contains most of the key components of the system, and incorporates the CPU, bus, memory, controllers, expansion slots, and so on. Memory chips may be added to it, and some motherboards allow for the CPU to be replaced. Also called motherboard, mainboard, or system board.